The present invention relates generally to evaporator installations, and more particularly to evaporators for processing radioactive liquid wastes in a nuclear power plant.
Until relatively recently the need for an evaporator system in nuclear power plants was confined to controlling the concentration of primary water, i.e. the liquid heat transfer medium between the atomic pile and the steam generators of the plant. This liquid, which normally contains sodium sulphate or boric acid, was sufficiently free of scale forming contaminants and sufficiently uniform in concentration that it could be evaporated without excessive scaling in a conventional calandria or long-tube-vertical (LTV) evaporator, wherein circulation through the heat exchanger was obtained by thermal gradients, as opposed to pumping or other mechanical circulation means.
It has recently been recognized in the operation of nuclear power plants that liquids accummulated from floor drainage systems and other internal reactor-related sources constitute a potential radiation hazard, and must therefore be handled and disposed of as radioactive waste material, typically by sealing the waste in concrete drums or barrels, and then burying it either on land or at sea. Because of the relatively high expense of the disposal process, it is highly desirable that the radioactive liquid waste be concentrated prior to being sealed in the containers for burial. To this end the waste may be first processed in one or more evaporator stages, wherein a large portion of the liquid is removed as harmless non-radioactive steam or water vapor leaving concentrated liquor for disposal.
Unfortunately, the concentrated waste liquor, which unlike primary water may contain suspended solids such as calcium, sodium and magnesium carbonates and sulphates in variable and unpredictable concentrations, is prone to boil and produce scaling in the heat exchanger and the other components of the conventional calandria and LTV natural flow evaporator systems heretofore used in the primary water concentrating process. This eventually reduces the efficiency of the evaporator to the point that its utility in the concentrating process is seriously impaired. While it is possible to remove some of the accummulated scaling by boiling out the evaporator system with a suitable solvent, it eventually becomes necesaary to shut down and dismantle the system so that insoluble scales and plugged tubes can be cleaned by mechanical means. Because the liquor within the evaporator systems is highly radioactive and the systems are therefore normally installed in concrete vaults for shielding and are not easily accessible, dismantling prior-art systems for descaling or repair has been a time-consuming and expensive operation.
While forced circulation (FC) evaporators, where a pump or other mechanical circulation means is provided to circulate the liquor, overcome the scaling problem to a large degree by keeping the liquor circulating at a controlled rate with carefully controlled temperature gradients within the heat exchanger whereby boiling is restricted to the evaporation chamber, the use of these evaporators for concentrating radioactive liquids has been avoided in the past because of their use of a pump. Heretofore, such pumps, which were arranged below the level of the liquid in the system and therefore necessarily required a shaft seal resistant to the passage of the fluid being pumped, required frequent servicing which could be accomplished only with great difficulty because of the surrounding radiation shielding and the high radiation levels of the liquor. Furthermore, the use of a pump was considered objectionable because it increased the volume requirements of the system and made necessary additional access passageways, thus significantly increasing the required radiation shielding around the evaporator.
Accordingly, it is a general object of the present invention to provide a new and improved evaporator system for concentrating radioactive liquid wastes in a nulcear power plant or the like.
It is a more specific object of the present invention to provide an evaporator system for a nuclear power plant or the like which can be more readily dismantled for descaling or repair.
It is a still more specific object of the present invention to provide a new and improved evaporator system which can be serviced without being drained.
It is another object of the present invention to provide an evaporator for a nuclear power plant which minimizes scaling and the need for periodic maintenance.
It is another object of the present invention to provide an evaporator system which can be serviced from one access plane.
It is another object of the present invention to provide a new and improved pump mounting and assembly for an evaporator system.
It is another object of the present invention to provide a circulation pump for an evaporator system which provides a minimal need for periodic maintenance.
It is another object of the present invention to provide a circulation pump for an evaporator system which can be serviced from one access plane without draining the system.